So, what is "the Philadelphia area"?
The easiest definition is to equate the area with the Delaware Valley. Here's a map.
The Delaware River runs through the map from the top center, takes a jag to the right by Trenton, and then doubles back to Wilmington where it runs into Delaware Bay. It includes parts of the states of Pennsylvania, New Jersey, Delaware, and Maryland.
The counties in the Delaware Valley are (with metropolitan areas listed, and the largest or most influential in bold and italics):
New Castle (Wilmington)
Also associated with the region:
Atlantic (Atlantic City)
Ocean (Ocean City)
Also associated with the region is the Lehigh Valley:
Lehigh (Allentown, Bethlehem)
Northampton (Bethlehem, Easton)
For the purposes of this blog, I will generally avoid Maryland and most of Delaware, but will include the Lehigh Valley and other nearby areas, including Lancaster County, since they play into the story of the Church in this region.
Map of the Delaware Valley from Wikipedia.